There exist different channel access methods for sending and receiving digital signals. In TDMA, Tire Division Multiple Access, a channel consists of a time slot in a periodic train of time intervals over the same frequency. In FDMA, Frequency Division Multiple Access, a communication channel as a single radio frequency band. Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. In contrast, CDMA, Code Division Multiple Access, allows signals to overlap in both time and frequency. Thus, several CDMA signals can share the same frequency band, but the CDMA receiver can also operate at several frequency bands.
Within the CDMA technique simultaneous connections can thus make use of a common frequency band. The selection, i.e. discrimination, between the des red signal and other signals is carried out by suitable signal processing, which is based on the desired signal being coded with a different code than other signals. The radio channels in a CDMA system are thus obtained by using different codes for each channel. Typically, the channels are obtained by using binary PN code sequences.
The transmitted information in the COMA radio signal is coded (spread) by a specific spreading code in the transmitter. At the receiving end, the coded information is decoded (despread) by correlating with the same specific spreading code again or by filtering the received information in a matched filter.
Second generation mobile network systems, wherein CDMA is used, such as IS-95, were primarily designed for transferring speech signals. The CDMA frequency band used in such systems was limited in comparison with the wide band CDMA system, WCDMA, which will be used in the third generation mobile system, UMTS, for transferring different services with signals of a wide spectrum of bit rates, such as speech, video and other services.
In WCDMA systems, user channels can be assigned binary code sequences of varying length based on the data rate of each channel. This allows for different services, e.g. speech, video, data etc. Orthogonal codes are codes that do not correlate to each other at a given time offset. Using such codes will therefore discriminate desired channels and the use of orthogonal codes will reduce the interference. Normally, the interference will not be completely eliminated as e.g. time dispersion will partly destroy the orthogonality between signals coded with orthogonal codes.
Different code structures exist, in which all available codes in a system are arranged, usually after the bit rate they provide. Examples of code structure systems used in WCDMA are e.g. the OVSF, Orthogonal Variable Spreading Factor, codes, explained in the following. One way of creating OVSF codes is by means of Walsh codes of different lengths, i.e. different spreading factors.
In the international patent application WO 9503652, user channels are assigned binary Walsh code sequences of varying length based on the data rate of each channel.
The lowest possible order (i.e. first order) of such a code, called a root code, has a code length of one bit, and may be equivalent to “0”. A tree of Walsh sequences may be envisioned as a set of interconnected nodes each having two branches, where all of the nodes may be traced back to the root node. Then the two branches from this root node will be connected to a pair of nodes defined by Walsh sequences “00” and “01”, which are called second order codes. This process can be continued by deriving a Walsh function matrix by branching the node “00” into the nodes “0000” and “0011”, and branching the node “01,” to “0101” and “0110” which are called third order codes. The Walsh sequence defining a node of the tree branching from a given node is not orthogonal to the Walsh sequence associated with that node. Therefore, associated nodes of different orders do not discriminate channels and branch-connected codes may not be simultaneously used. Any other Walsh sequences defining nodes not connected to the given node can be simultaneously used as codes to define other mobile channels.
The amount of codes to be allocated is thus restricted, as the number of available codes for a specific code length is mathematically limited. OVSF codes are downlink channelisation codes used in WCDMA that preserves the orthogonality between channels of different rates and spreading factors. The code structure of the OVSF codes are described by means of a code tree structure, which is illustrated in FIG. 1 and will be described in more detail later on. In the code tree structure services requiring codes of greater length, like speech, are to the right of the code tree, whereas more requiring services (i.e. requiring higher data rates), like video, need codes of shorter lengths as seen to the left in the code tree. In cell able to provide different kinds of services, codes of different spreading factors and rates will be requested, corresponding to different services.
Prior art solutions for allocation of codes in CDMA systems, able to provide speech and other services, in which the codes to be allocated are selected systematically, do not seem to exist as far as known. If the allocation of codes is performed by just signing the first available free code without any further rules, it represents the simplest way to allocate such code requests, the so called sequential mode.
Problems in using such sequential code allocation methods in wide band systems using several code levels (orders) arise upon releasing the used codes when older calls expire, which results in holes in the code structure of busy codes. Even if the maximum total bit rate available has not been reached (i.e. there are still some free codes), new high bit rate call requests might rot be satisfied unless used lower bit rate codes are reallocated in order to free a higher bit rate code of requested level. Since reallocations need signalling between the base stations and the involved mobile stations, critical situations might arise in high load conditions.
In the international patent application WO 95/03652 (QUALCOMM INCORPORATED), proposing a code allocation system for wide band CDMA systems, this problem has been discussed and the idea of minimizing the number of disqualified shorter-length codes is presented. The document states the opportunity or assigning codes that are related to busy codes, in order to minimize the fragmentation of the tree and suggest to allocate slow codes that are related to unavailable fast codes. Furthermore, reallocation is presented to increase the availability of fast codes. A precise algorithm to achieve this result as, however, not presented and the problem of future availability is not taken into account. Thus, there is a need for such an algorithm,.
One object of the invention is to develop a method, which minimizes the signalling in the system when allocating codes in a multi speed system.
Another object is to reduce the setup delay when allocating new calls.
A third object is to develop a method, which maximizes the amount of available free codes at different levels.
A fourth object is to develop a method for reallocation when no free codes are available, while minimizing the need for such reallocation.
A fifth object of the invention is to develop a method attaining that a minimum of higher-rate codes become unavailable by using an algorithm to carry out the allocation of the invention.